Make max_align_t respect _Float128

[Joseph Myers <joseph at codesourcery dot com>]

The _FloatN, _FloatNx, _DecimalN and _DecimalNx types are specified in
such a way that they are basic types, meaning that max_align_t must be
at least as aligned as those types.

On 32-bit x86, max_align_t is currently 8-byte aligned, but
_Decimal128 and _Float128 are 16-byte aligned, so the alignment of
max_align_t needs to increase to meet the standard requirements for
these types.

This patch implements such an increase.  Because max_align_t needs to
be usable for C++ as well as for C, <stddef.h> can't actually refer to
_Float128, but needs to use __float128 (or some other notation for the
type) instead.  And since __float128 is architecture-specific, there
isn't a preprocessor conditional that means "__float128 is available"
(whereas one could test __FLT128_MANT_DIG__ to see if _Float128 is
available).  But I believe the only case that actually has an
alignment problem here is 32-bit x86, and <stddef.h> already has lots
of conditional specific to particular architectures or OSes, so this
patch uses a conditional on __i386__.  If any other architectures turn
out to have such an issue, it will show up as failures of one of the
testcases added by this patch.

Such an increase is of course an ABI change, but a reasonably safe
one, in that max_align_t doesn't tend to appear in library interfaces
(rather, it's something to use when writing allocators and similar
code; most matches found on codesearch.debian.net look like copies of
the gnulib stddef.h module rather than anything actually using
max_align_t at all).

(I think glibc malloc alignment should also increase to 16-byte on
32-bit x86 so it works for allocating objects of these types, which is
now straightforward given the fix made for 32-bit powerpc.)

Bootstrapped with no regressions on x86_64-pc-linux-gnu, and
spot-tested with -m32 that the new float128-align.c test now compiles
where previously it didn't.  OK to commit?

gcc:
2016-08-26  Joseph Myers  <joseph@codesourcery.com>

	* ginclude/stddef.h (max_align_t) [__i386__]: Add __float128
	element.

gcc/testsuite:
2016-08-26  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/float128-align.c, gcc.dg/float128x-align.c,
	gcc.dg/float16-align.c, gcc.dg/float32-align.c,
	gcc.dg/float32x-align.c, gcc.dg/float64-align.c,
	gcc.dg/float64x-align.c, gcc.dg/floatn-align.h: New tests.

Index: gcc/ginclude/stddef.h
===================================================================
--- gcc/ginclude/stddef.h	(revision 239784)
+++ gcc/ginclude/stddef.h	(working copy)
@@ -426,6 +426,14 @@
 typedef struct {
   long long __max_align_ll __attribute__((__aligned__(__alignof__(long long))));
   long double __max_align_ld __attribute__((__aligned__(__alignof__(long double))));
+  /* _Float128 is defined as a basic type, so max_align_t must be
+     sufficiently aligned for it.  This code must work in C++, so we
+     use __float128 here; that is only available on some
+     architectures, but only on i386 is extra alignment needed for
+     __float128.  */
+#ifdef __i386__
+  __float128 __max_align_f128 __attribute__((__aligned__(__alignof(__float128))));
+#endif
 } max_align_t;
 #endif
 #endif /* C11 or C++11.  */
Index: gcc/testsuite/gcc.dg/float128-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float128-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float128-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float128 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float128 } */
+/* { dg-require-effective-target float128 } */
+
+#define WIDTH 128
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float128x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float128x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float128x-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float128 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float128x } */
+/* { dg-require-effective-target float128x } */
+
+#define WIDTH 128
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float16-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float16-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float16-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float16 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float16 } */
+/* { dg-require-effective-target float16 } */
+
+#define WIDTH 16
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float32-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float32-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float32-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float32 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float32 } */
+/* { dg-require-effective-target float32 } */
+
+#define WIDTH 32
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float32x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float32x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float32x-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float32 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float32x } */
+/* { dg-require-effective-target float32x } */
+
+#define WIDTH 32
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float64-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float64-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float64-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float64 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float64 } */
+/* { dg-require-effective-target float64 } */
+
+#define WIDTH 64
+#define EXT 0
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/float64x-align.c
===================================================================
--- gcc/testsuite/gcc.dg/float64x-align.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/float64x-align.c	(working copy)
@@ -0,0 +1,9 @@
+/* Test _Float64 alignment.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+/* { dg-add-options float64x } */
+/* { dg-require-effective-target float64x } */
+
+#define WIDTH 64
+#define EXT 1
+#include "floatn-align.h"
Index: gcc/testsuite/gcc.dg/floatn-align.h
===================================================================
--- gcc/testsuite/gcc.dg/floatn-align.h	(nonexistent)
+++ gcc/testsuite/gcc.dg/floatn-align.h	(working copy)
@@ -0,0 +1,18 @@
+/* Tests for _FloatN / _FloatNx types: test max_align_t alignment.
+   Before including this file, define WIDTH as the value N; define EXT
+   to 1 for _FloatNx and 0 for _FloatN.  */
+
+#define CONCATX(X, Y) X ## Y
+#define CONCAT(X, Y) CONCATX (X, Y)
+#define CONCAT3(X, Y, Z) CONCAT (CONCAT (X, Y), Z)
+
+#if EXT
+# define TYPE CONCAT3 (_Float, WIDTH, x)
+#else
+# define TYPE CONCAT (_Float, WIDTH)
+#endif
+
+#include <stddef.h>
+
+_Static_assert (_Alignof (max_align_t) >= _Alignof (TYPE),
+		"max_align_t must be at least as aligned as _Float* types");

-- 
Joseph S. Myers
joseph@codesourcery.com